Just download a few and see how they act, I know for sure a lot of them don't flip the clock because in the faq's at the start of the page the user has why won't the clock flip? & goes on to explain why they don't.
You will need to download these to your sd card so they will show up as an .apk so you can install them other wise they will download as a .bin file if you use your phones browser. Even if they download as a.bin you can rename the file to whatever you want like glassskin.apk and it will work as long as you put .apk at the end.
Hope that helps!